RFID read-write devices (reader devices for short), known to one skilled in the art under the term dense reader environment, are increasingly being used in large numbers in confined spaces, and they control production and product flows. Reader devices are networked to a control processor (controller for short), primarily via LAN connections, and more rarely via WLAN ones. The controller, also designated as an edgeware controller, is the connecting point to the applications which run on the computer of an operator (also designated as middleware software), with these computers representing queried data contents. The controller has three main tasks to carry out, first governing and controlling the number of reader devices, secondly translation of high-level tasks of middleware into instructions to the reader devices for querying the data contents of electronic labels (labels for short) and third, further processing of data contents of the labels of numerous reader devices to obtain the desired information for the middleware. Critical points in this regard are, for example, the mutual interferences of the reader devices, which operate with large sending capacity, the large numbers of data packets that run via the communication network to the controller and the loading of the communication network by other processes of the operator. Various methods have been proposed to defuse one or another of these problems.
Thus, US 2006 0279406 proposes a synchronization of reader devices, conducted by a master station, which delivers a time signal on a separately wired synchronization bus; true, it has high availability, but involves additional expense for hardware and cables for it.
US 2007 0001813 uses a central controller for synchronization of reader devices, thus eliminating the additional timing line. The controller forms groups of reader devices which are not mutually interfering and then works according to a temporal sequence, a schedule. These measures are also known to one skilled in the art as SDMA and TDMA. This method, like others, is suited to produce a static synchronization of the reader devices, but entails high availability of the communication network, because the schedule is filed in the controller.
A proposal is made in US 2005 0088284 that an interference list be kept in readiness in each reader device, with this interference list being able to prevent a simultaneous, interfering operation of neighboring reader devices. Via a signalization on the network, active reader devices communicate the start and end of a sending phase to the neighboring reader device. They thus permit a form of listen before talk technique, so that owing to decisions in each reader device, interferences also can be avoided.
EP 1762960 depicts a process for operating multiple reader devices in which various communication protocols can be used, such as differing modulation, frequency, coding, data rate, so that no interferences occur. On the other hand, for this in each reader device a schedule is stored in a storage device. The synchronization is effected either via a master or via a coordinator having an activity monitor. The reader devices can communicate among themselves by means of an activity signal to prevent interferences. The proposed solution is primarily aimed at applications with two different protocols, such as near field communication (NFC) or a contactless smart card, which both, for example, can be found in a cell phone or a PDA, and would interfere with each other.
In WO 2007 078440, a certain degree of intelligence for optimizing an RFID network (not to be confused with an LAN network between the controller and the reader device) is put into a reader device. The RFID network is designed without a controller and designs itself, according to a known art, the so-called ad hoc networks, in the area of the wireless sensor net. Instead of the controller, a primary and multiple secondary reader devices are included, with the primary reading devices assuming part of the tasks of the controller. The performance capability of a primary reading device cannot in any case be equal that of a controller, and therefore in large networks, it soon reaches its limits. Here limits are set primarily by large RFID networks such as availability, redundancy, error tolerance and load compensation. A storage block with RFID network rules prevents mutual interferences in that the primary reader device governs the frequency and time-slot resources as network rules, and gives assignments to the secondary reader devices. Additionally, an optimization module is available which can statistically and logically process data of the electronic labels and include planned processing of other stations into the activity, which primarily is helpful with processing of an electronic label by multiple stations. The stations organize themselves, especially in a version that even does away with a primary reading device. This method has a drawback in that convergence and stable operation cannot readily be assured and are not able to be much influenced, since the RFID network behaves very dynamically and, as recorded in WO 2007 078440, similar to a neural net. Reader devices can also communicate with each other via the same antenna as for the processing of the electronic labels.
An additional communication option via the air interface with synchronization of reader devices and a procedure based thereupon is carried out in EP 1719067 by means of the so-called reader service signals.
US 2007 0046467 shows a network which connects reader devices in a chain to a controller (there designated as a server). The first reader device communicates with the server and a second reader device, which in turn is in connection with a further reader device. To have available a time signal for a synchronization of the read-write cycles in each reader device, they periodically obtain the time from a so-called network time protocol (NTP) server or from a clock in a first reader device, which is attached to the server.
EP 1672592 clearly depicts the task of a controller for scalable, large RFID networks. In particular, examples of data processing of read-out labels are implemented. Virtually the entire task is given to the controller of governing the reader devices and processing data. Via a user-user interface, application programs—also designated as a configuration—for processing of labels are entered into the controller (middleware interface). Using the data read by the selected reader devices, the processing then correspondingly occurs in the controller. The controller individually governs every action of the reader devices, such as antenna selection.
What is common in the noted prior art is that an RFID network with large and increasing numbers of reader devices to be governed and a growing number of labels to be read comes into contact with processing limits and thus the readout rate is limited by the availability of the communication network and/or by the also concomitant interferences on the air interface. The communication network is often part of the infrastructure of a user, and a great variety of processes run on it. The result of this is that a communication network between the reader device and controller connecting it is not available for this reader device, or that such a communication network has too little capacity to send all of the data read from labels to the controller in timely fashion. It should be mentioned at this point that upon being queried, each label for the most part is detected multiple times, as long as it is in the field of one or more reader devices. Simple, i.e., uncoordinated, transfer of all this data by conventional, unintelligent reader devices generally does not make sense, least of all in large RFID networks.